Method of allocating a communication channel in a satellite network

ABSTRACT

The invention concerns a method of allocating a respective free radio beam channel to calling and called stations of a satellite telecommunication network. If there is no free channel on a beam forming the coverage including one given station, the method of the invention looks for free channels in each beam adjacent said beam, and allocates one of the adjacent beam free channels to the given station as soon as in an optimal subset of beams comprising all beams using the same channel as the free channel each level of isolation equal to the ratio of an antenna gain of a main lobe of a respective one of the beams of the optimal subset to the sum of the maximal antenna gains of secondary lobes of the other beams is greater than a given threshold.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to telecommunications betweenearth transmitting-receiving stations via a satellite.

2. Description of the Prior Art

More exactly, the invention relates to a method of allocatingcommunication channels to calling stations and called stations in asatellite telecommunications network employing frequency re-use inresponse to a call setting-up request transmitted by the calling stationto a control station SC. As shown in FIG. 1, a satellitetelecommunication network is typically defined by a plurality N=9 ofearth transmitting-receiving stations S1 through S9, the control stationSC and a satellite SA. Setting up a call between a calling station, suchas the station S3, and a called station, such as the station S6, of theplurality of stations entails allocating a channel or frequency band B3to the calling station S3 and a channel B6 to the called station S6,each of the channels B3 and B6 belonging to a given beam. The data to betransmitted produced by the station S3 is transmitted to the satelliteSA on the uplink channel B3 and retransmitted from the satellite SA tothe called station S6 on the downlink channel B6.

A call setting-up phase is started at the initiative of the callingstation S3 which produces a call setting-up request message intended forthe control station SC via the satellite SA. This call setting-uprequest message is sent on an uplink signalling channel BS from thecalling station to the satellite and retransmitted on a downlinksignalling channel BS from the satellite to the control station SC. Thecontrol station SC retransmits a call setting-up message to the callingstation S3 as soon as two channels, the channels B3 and B6 in thisexample, can be respectively allocated to the stations S3 and S6.

The prior art provides two methods of managing radio beam coverage in asatellite network in the context of a low-power satellite SA.

In a first method, the network having a predetermined number ofchannels, a variable number of channels are allocated to each beamaccording to the level of traffic. To this end dynamic reallocation ofchannels of the payload of the satellite SA between the radio beams iseffected, depending on the traffic demand on those beams. In a method ofthis kind the beams are fixed, which means that the coverage of eachbeam for each of the channels constituting it has a constant extentdefined by an antenna gain at less equal to a minimal gain. On the otherhand, for a given beam, the number of channels varies in time. Reducingby one the number of channels in one beam entails increasing by one thenumber of channels in another beam. A satellite implementing this methodis called a capacity reallocation matrix satellite.

A second method is described in U.S. Pat. No. 5,355,138 issued on Oct.11, 1994, the contents of which are hereby incorporated by way ofreference into this description. A satellite network is managed bycoverage reconfiguration. In this second method, unlike the first methodpreviously mentioned, the number of channels in each radio beam is fixedand the coverage of each beam is modified in time by reducing orincreasing the coverage area in accordance with call setting-up requestsproduced in the network. In practise, although the number of channels ineach beam is constant, there is provision for exchanging channelsbetween beams. Allocating a channel of a first beam to a second beamentails allocation to the first beam of a channel of the second beam.Accordingly, the "bandwidth" of the set of channels of a beam remainsconstant. The size of the beam coverage is reconFig.d in the followingmanner.

To set up a call, i.e. a link, between a calling station and a calledstation of the network via the satellite SA, the calling station firstsends a call setting-up request message to the control station SC via asignalling channel BS of the satellite SA. The development in theconfiguration of the network relative to the calling and called stationsis simulated in the control station. A call setting-up authorizationmessage is transmitted by the control station SC to the calling stationwhen simulated authorizations have been derived by the control stationSC for both the calling and called stations. Each of these simulatedauthorizations corresponds to the possible allocation of a respectivefree channel for the calling station and the called station.

Two main scenarios are provided for each station in the simulation inthe control station. Each of these two scenarios is directly related tothe limited power of the satellite SA.

In the first scenario (FIG. 5 of U.S. Pat. No. 5,355,138) relating tothe situation in which the calling or called station does not belong toany radio beam coverage, the coverage nearest the station is selectedfirst. The beam coverages other than the selected coverage are thenreduced in size to free up a fraction of the power output by thesatellite SA in these other beams. This freed up portion of the power isthen used to increase the surface (footprint) of the selected coveragein order to include the station in it. A simulated authorization isderived in the control station as soon as the station can be included inthe selected coverage by increasing its surface and a channel is free inthe selected beam coverage.

In the second scenario (FIG. 11 of U.S. Pat. No. 5,355,138), relating tothe situation in which the calling or called station is included in aradio beam coverage, there are two sub-scenarios. Either the beam ofthis coverage has a free channel, in which case this free channel isallocated to the station; or there is no free channel in the beamassociated with the coverage including the station, in which case thecoverage nearest this coverage having a free channel is looked for. Aload transfer is then effected between the nearest coverage having afree channel and the coverage including the station. This load transferentails increasing the surface of the nearest coverage in order toinclude the station in it and commensurate reduction in the surface ofthe coverage including the station. A simulated authorization is derivedfor the station as soon as it can be included in the adjacent coverageand a channel is free in the adjacent coverage.

As soon as simulated authorizations have been derived in the controlstation SC for both the calling and called stations, the control stationsends to the satellite SA power and phase-shift control values to modifythe geometries of the coverages concerned, that is to say, also, thoseincluding the calling and called stations in accordance with thesimulation carried out. A call setting-up authorization message is alsosent via the satellite SA to the calling station in order that acommunication phase between the calling and called stations can begin.

In a variant of the first scenario, relating to the situation in whichthe station is not included in any coverage, there is provision forselecting not only the nearest coverage but also the lowermost surfacecoverage in the network and for each of these two coverages to reducethe surfaces of the other coverages in order to include the station inthe selected coverage, either the nearest one or the one with thelowermost surface. In this variant, two respective gains for the stationare calculated by simulation in the control station SC according towhether the station is included in the nearest coverage or the lowermostsurface coverage. The coverage to include the station S is chosen asthat which offers the highest gain. The power and phase-shift controlvalues for the radiating elements of the satellite antenna are thustransmitted by the control station SC to the satellite SA.

The first and second prior art methods described hereinabove ignorefrequency re-use in the network and therefore the problem caused bymanagement of channels corresponding to the same frequency band indifferent beams. For reasons relating to limitation of the frequencybandwidth available on the satellite, it can be beneficial to re-use thesame channels or frequency bands in different beams.

Nevertheless, it seems that such re-use of the same band of frequenciesin different beams causes interference dependent on the angularseparation between the different beams.

OBJECT OF THE INVENTION

The invention is directed to providing a method for managing theallocation of a channel to a station which guarantees that channels ofdifferent radio beams corresponding to the same frequency band do notcause more than a specific maximum level of interference.

SUMMARY OF THE INVENTION

In a first variant, the invention concerns a satellite network usingcoverage reconfiguration.

Accordingly, in a satellite telecommunication network comprising acontrol station, a satellite for forming beams of radio channels underthe control of the control station, and plural earth stations fortransmitting beam channels to the satellite and receiving beam channelsfrom the satellite, same channels being re-used in different beams,there is provided a first method for allocating a free beam channel bythe control station to calling and called stations, in response toreception of a call setting-up request message emitted by the callingstation via a signalling channel of the satellite in the controlstation.

The first method entails simulation in the control station of one offirst and second alternatives separately for each of the calling andcalled stations, and then a final step.

(a) The first alternative whereby at least one free channel belongs to abeam forming a coverage including each of the calling and calledstations, comprises the step of allocating the at least one free channelto each of the calling and called stations.

(b) The second alternative whereby there is no free channel on the beamforming the coverage including each of the calling and called stations,comprises the following iterative steps:

(b1) searching for free channels in each of adjacent beams from thatnearest to that farthest from the beam forming the coverage includingeach of the calling and called stations thereby identifying freechannels, and

(b2) in response to the free channels identified in each of the adjacentbeams, allocating in a simulated way one of the free channels identifiedin each of the adjacent beams to each of the calling and called stationsby

transferring load between the coverage including each of the calling andcalled stations and an adjacent coverage associated with each of theadjacent beams by reducing in surface the coverage including each of thecalling and called stations and increasing in surface the adjacentcoverage so that each of the calling and called stations is included inthe adjacent coverage, and

determining optimal subsets of beams each comprising beams able tore-use the same channel, and selecting the one of the free channelsidentified in each of the adjacent beams as that which is least used inone of the optimal subsets of beams including each of the adjacentbeams.

(c) The final step comprises the step of emitting a call setting-upauthorization message from the control station to the calling station assoon as the one of the free channels identified is allocated to each ofthe calling and called stations.

Preferably, the step of determining optimal subsets of beams includesthe following iterative steps of:

constituting ##EQU1## separate subsets of T beams from I beams of thenetwork, T being an integer initialized to 2 and being incremented by 1on each iterative step up to at most I, and

for each of the C_(I) ^(T) subsets of T beams calculating T levels ofisolation each equal to the ratio of an antenna gain of a main lobe of arespective one of the beams in each of the C_(I) ^(T) subsets of T beamsto the sum of the respective maximal antenna gains of secondary lobes ofthe other beams in each of the C_(I) ^(T) subsets of T beams providedthat a conditional relationship establishing that T levels of isolationcalculated for at least one of the C_(I) ^(T) subsets of T beams aremore than a predetermined threshold is satisfied.

The optimal subsets of beams is made up of subsets of beams from C_(I)^(T).sbsp.L⁻¹ =(I!)/[(T_(L) -1)!(I-T_(L) +1)!] for each of which [T_(L)-1] levels of isolation calculated are more than the predeterminedthreshold. T_(L) denotes an integer value assumed by T for which theconditional relationship is not satisfied.

Advantageously, the first method can include in the simulation carriedout in the control station, a third alternative whereby each of thecalling and called stations is not included in any beam coverage. Thethird alternative comprises the steps of:

determining from beam coverages existing in the network a coveragenearest to each of the calling and called stations and a lowermostsurface coverage;

by iteration, for each of the nearest and lowermost surface coverages,reducing in size each of the coverages other than each of the nearestand lowermost surface coverages whilst maintaining in the coveragesstations which are active prior to the reception of the call setting-uprequest message, until each of the nearest and lowermost surfacecoverages can be increased to include each of the calling and calledstations with a respective antenna gain in each of the nearest andlowermost surface coverages, and

selecting one of the nearest and lowermost surface coverages into aselected coverage as a function of the higher of two antenna gainsrespectively calculated for the nearest and lowermost surface coveragesto include each of the calling and called stations in the selectedcoverage,

the step of selecting being followed by one of the first and secondalternatives and the final step.

Furthermore, in order to uniformly distribute the busy radio channelsinto the beams of the network, the first method comprises, in responseto a number of busy radio channels in one of the beams existing in thenetwork greater than the integer part of a ratio of a number of busyradio channels in the network to the number of beams existing in thenetwork, executing the second alternative for each station covered bythe one of the beams, when the number of the busy radio channels in theone of the beams is greater than the integer part, so that to each ofthe stations belonging to a coverage of the one of the beams is assigneda radio channel of a beam adjacent one of the beams to free up a radiochannel in the one of the beams.

In a second variant, the invention concerns a satellite network usingcapacity reallocation.

Accordingly, there is provided a second method for allocating a freebeam channel by the control station to calling and called stations, inresponse to reception of a call setting-up request message emitted bythe calling station via a signalling channel of the satellite in thecontrol station. The second method entails also simulation in thecontrol station of one of first and second alternatives separately foreach of the calling and called stations, and then a final step. Thefirst alternative, the first substep (b1) in the second alternative andthe final step in the first and second methods are identical.

The second method differs from the first method by a second substep (b2)which consists in allocating to each of the calling and called stationsof one of the free channels identified in each of the adjacent beams assoon as in an optimal subset of beams comprising all beams using thesame channel as one of the free channels, each level of isolation equalto the ratio of an antenna gain of a main lobe of a respective beam fromthe all beams in the optimal subset to the sum of maximal antenna gainsof secondary lobes of the other beams in the optimal subset is greaterthan a given threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will be apparent moreclearly from the following description of several embodiments of theinvention with reference to the corresponding accompanying drawings inwhich:

FIG. 1, already commented on, shows a satellite network installationincluding a satellite and a plurality of earth stations;

FIG. 2 shows two antenna radiation diagrams to illustrate the phenomenonof interference between channels of two beams;

FIG. 3 is a general algorithm for determining optimum subsets of beamswith tolerable interference;

FIG. 4 is an algorithm for allocating a channel in a satellite networkwith frequency re-use employing reallocating capacity between beams; and

FIG. 5 is an algorithm for allocating a channel in a satellite networkwith coverage reconfiguration employing frequency re-use.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 2 are shown two antenna radiation diagrams, respectively insolid line and in dashed line. FIG. 2 is a plot of the gain G as afunction of an angle θ. These two radiation diagrams are associated withrespective radio beams F1 and F2 in the satellite network shown in FIG.1 and are intended to show the cause of interference between channelscorresponding to the same frequency band in different beams. Each of thetwo radiation diagrams has a respective main lobe LP1, LP2 andrespective secondary lobes LL1, LL2 to the side of the main lobe. If twobeams, here denoted F1 and F2, using two respective channelscorresponding to the same frequency band are not sufficiently isolatedspatially, channel interference occurs between the secondary lobes LL1,LL2 of one beam and the main lobe LP2, LP1 of the other beam, for thechannel in question. Accordingly, data transmitted on the channel of onebeam is disrupted by the data transmitted on the channel of the otherbeam. It is assumed in practise that the beams are sufficiently isolatedspatially in pairs for only the secondary lobes of a beam to have to betaken into account in calculating the disturbance caused to the mainlobe of another beam.

Of course, such interference affects not only the uplink channels fortransmitting data from the earth stations to the satellite SA but alsothe downlink channels for transmitting data from the satellite SA to theearth stations. If two beams F1 and F2 are associated with tworespective channels corresponding to the same frequency band, the uplinkdata received by the satellite on one of the two channels can bedisturbed by data received on the other of the two channels, dependingon the geographical locations of the two stations sending on therespective channels of the two beams F1 and F2. If two beams areassociated with two respective channels corresponding to the samefrequency band, the downlink data on one of the two channels receivedfrom the satellite SA by a station covered by one of the beams isdisturbed by data transmitted by the satellite on the other of the twochannels to a station covered by the other of the beams, depending onthe geographical locations of the two stations.

As shown in FIG. 2, there are in practise two levels of isolation fortwo beams associated with respective channels corresponding to the samefrequency band. To calculate each of these two levels of isolation,respectively representative of the minimal quality of the link on thetwo channels, independently of the location of the stations, each of thetwo channels associated with the two beams F1 and F2 is considered inturn as a disturbed channel and a disturbing channel. In FIG. 2, a firstlevel of isolation relating to isolation of the channel of beam F2relative to the channel of beam F1 is equal to the ratio of an antennagain, called the minimal gain G_(LP2), of the main lobe LP2 at theboundary of the coverage of the beam F2 to a maximum antenna gainG_(LL1) of the secondary lobes LL1 of the beam F1. A second level ofisolation relating to the isolation of the channel of beam F1 relativeto the channel of beam F2 is equal to the ratio of an antenna gain,called the minimal antenna gain G_(LP1), of the main lobe LP1 at theboundary of the coverage of the beam F1 to a maximal gain G_(LL2) of thesecondary lobes LL2 of the beam F2. More generally, in the case of Nbeams associated with N respective channels corresponding to the samefrequency band, N levels of isolation are determined by calculating inturn the disturbance caused on one of the N channels of one of the beamsby the (N-1) other channels of the other beams. Each of the N levels ofisolation is equal to the ratio of the minimal antenna gain of the mainlobe of one of the beams at the boundary of the beam concerned to thesum of the maximal antenna gains of the secondary lobes of the otherbeams.

The general algorithm for determining optimal subsets of beams based onthe foregoing considerations is now described with reference to FIG. 3.Two variant implementations of this algorithm or steps of this algorithmfor implementation of the invention are described subsequently withreference to FIGS. 4 and 5, respectively for a satellite network withfixed coverages using capacity reallocation and for a satellite networkusing coverage reconfiguration. The objective of an algorithm of thiskind, used in simulation in the control station SC, is to determineoptimum subsets of beams, i.e. to determine groups of beams which caneach re-use channels corresponding to the same frequency band withoutgenerating interference between beams and each include a maximal numberof beams. The algorithm determines these optimum subsets by aniteration.

It should be noted that in the following description the calculation oflevels of isolation between beams is of the "symmetrical" type for theuplinks and downlinks. This means that two levels of isolation of adisturbed beam channel relative to a plurality of disturbing beamchannels, of uplinks and downlinks, respectively, are equal. This"symmetrical" type simplifies the calculations to be carried in thecontrol station.

In practise an accurate calculation of a level of isolation of a beamrelative to another for two stations re-using two channels correspondingto the same frequency band would require a knowledge of the geographicallocation of the two stations respectively in the coverages of the twobeams in order to deduce therefrom antenna gains. For simplicity, aminimal level of isolation is calculated as a function of the main lobegain G for a first beam, at the boundary of that beam and a maximalsecondary lobe gain G of a second beam or the sum of plural maximalsecondary lobe gains G for plural second beams.

As shown in the initialization step EI0 and the main step E1 of FIG. 3,the algorithm starts by determining all subsets of size T=2, i.e. of twobeams, which can be formed from the I radio channel beams of thesatellite network. Combinatorial logic teaches that C_(I) ^(T=2) subsetswith two beams are determined with ##EQU2## M subsets determined in thisway are denoted SE(1) through SE(M=C_(I) ^(T=2)) in the algorithm.

For each of these subsets SE(m) with m between 1 and M=C_(I) ^(T=2), asshown by a step EI2 of initializing to 1 the index m and a step EI5 ofincrementing by 1 this index m, a calculation CNI of T levels ofisolation is effected. For each SE(m) of the subsets with T=2 beams,each of the T=2 levels of isolation N(1) and N(2) is compared to athreshold TH (step ET0). Each of the levels of isolation N(t), with1≦t≦T=2, is equal to the ratio of the gain GP(t) of the main lobe of therespective one of the beams in the subset SE(m) at the boundary of thecoverage of this beam to the sum of the respective maximal gains##EQU3## of the secondary lobes of the k other beams, with k≠t and k<T.For T=2 the aforementioned sum comprises only one gain value.

As indicated by the cyclic test steps ET0 and ET1 and the incrementingstep ET4, if the T levels of isolation N(t) with 1 ≦t≦T calculated forthe subset SE(m) are greater than the threshold TH, then T=2 respectivechannels corresponding to the same frequency plan can be re-used in thetwo beams constituting the subset SE(m) (step E3). On the other hand, ifat least one level of isolation N(t) is less than the threshold TH, thenre-use of channels corresponding to the same frequency band in the beamsof the subset SE(m) concerned is not allowed (step E4). As shown by thesteps ET2 and EI5 at the exit from step E3 and by the steps ET3 and EI7at the exit from step E4, a calculation CNI of two levels of isolationis effected for each of the C_(I) ^(T=2) subsets of T=2 beams of thenetwork. The characteristics of the subsets which can re-use channelscorresponding to the same frequency bands are memorized.

Another variable NS is initialized to zero at the start of thealgorithm, in step EI1, and indicates the number of subsets of size T=2that can use the same channel. This variable is incremented by one atthe exit from step E3, in step E14.

After all the subsets of size T=2 have been processed to determine whichones of these subsets contain beams authorized to re-use respectivechannels corresponding to the same frequency band, one of twoalternatives has to be selected. Either (step ET5 : yes) at least onesubset and at most M subsets containing T=2 beams can use two respectivechannels corresponding to the same frequency band have been determined,and the preceding steps for subsets of size T=T+1=3 (step EI8=step EI6)are iterated to determine which of these subsets of size 3 containingbeams able to re-use respective channels corresponding to the samefrequency band, by iteration for subsets of size T=T+1, with T≦I. Or(step ET5: no) no subset of size T containing beams which can re-use thesame frequency band has been determined and in this case (step EF) theconditions for channel re-use are restricted to subsets of size t=(T-1)which can re-use the same channel. This determines subsets which are"optimal" in the sense that they constitute groups of (T-1) beams ofmaximal size that can re-use respective channels corresponding to thesame frequency band. In practise, the intersections between subsets arenot necessarily empty, and the same respective channels that can be usedin the subsets of size (T-1) are therefore separate.

The FIG. 3 algorithm is a general algorithm relating to implementationof the invention. It is applied in part or in full depending on whetherthe satellite network uses capacity reallocation or coveragereconfiguration, respectively.

The FIG. 4 algorithm concerns a satellite network using capacityreallocation. In a network of this kind the beams are fixed and thecoverages of the beams are therefore of constant size. Networkmanagement consists in dynamic reallocation of channels between beams,the latter each containing a number of channels which varies with thetraffic in the beam coverage in question. The FIG. 4 algorithm isimplemented in the control station SC in response to reception of a callsetting-up request message from a calling station, denoted S_(k),wishing to set up a call to a called station denoted S_(l). For the callto be set up between the station S_(k) and the station S_(l), a channelmust be allocated to each of the stations in the beam defining thecoverage in which the station is located. Accordingly, as indicated instep E10, the FIG. 4 algorithm concerns both the calling station S_(k)and the called station S_(l), both denoted S in the remainder of thealgorithm.

A first test step ET10 looks for at least one free channel in the beam Fdefining the coverage in which the station S is located. If there is afree channel then it is allocated to the station S (step EF1). If not,successive beams Fa defining respective coverages from that nearest tothat farthest from the coverage of the beam F including the station Sare checked to see if there is at least one free channel (steps E11 andET11). If there is no free channel in any of the successive beams Fafrom that nearest to that farthest from the beam F defining the coverageincluding the station S, the algorithm is iterated for the next beam.

As soon as at least one free channel is found in one of these beams, inaccordance with the invention, the step involving calculation CNI of thelevels of isolation previously described with reference to FIG. 3 iscarried out. This step, denoted E12 in FIG. 4, is applied to a subsetSE(m) combining the beams using a channel corresponding to the samefrequency band as the free channel found in one of the beams Fa adjacentthe beam F defining the coverage including the station S. For theresulting subset SE(m), containing T beams, for example, T levels ofisolation N(t) are calculated, with 1≦t≦T. As shown in block CNI in FIG.3, each of these T levels of isolation N(t) is equal to the ratio of theantenna gain of the main lobe of the respective one of the beams of thesubset taken at the boundary of the coverage of that beam to the sum ofthe respective maximal antenna gains of the secondary lobes of the otherbeams in the resulting subset SE(m). If each of these T levels ofisolation N(t) is more than the threshold TH, then the free channelfound in said one of the adjacent beams is allocated to the beamdefining the coverage including the station S (step EF2).

If at least one of these T levels of isolation is less than thethreshold TH, the search for a free channel continues by iteration inthe subsequent adjacent beams. As soon as a free channel is found, thestep E12 for calculating the levels of isolation is carried out, and soon in an iterative manner, as shown by the connection between steps E12and E11 in FIG. 4, for as long as the levels of isolation calculated forthe subset containing the beams using the same frequency band as thefree channel found are not all more than said threshold TH.

In the foregoing variant of the invention relating to a satellitenetwork using capacity reallocation, it is assumed that the coverages ofthe beams are fixed. In a network using coverage reconfiguration thenumber of channels in each beam is constant but the geometry of thecoverages is variable. The use of a method of allocating a channel forthe variant of the invention relating to satellite networks of this kindusing coverage reconfiguration is described next with reference to FIG.5.

As previously, for the variant of the invention relating to a satellitenetwork using capacity reallocation, the FIG. 5 algorithm concerns boththe calling station S_(k) and the called station S_(l) in the networkand is implemented in the control station SC. In the remainder of thealgorithm from step E20 the calling station S_(k) and the called stationS_(l) are both denoted S. Step ET20 first determines whether station Sbelongs or does not belong to a coverage C of a beam F.

If the station S belongs to a beam coverage C, step ET21 then determinesif the beam associated with this coverage includes at least one freechannel. If there is a free channel, then this free channel is allocatedto the station S. If a plurality of free channels are found in the beamwhose coverage includes the station S, then to the station S isallocated whichever of those free channels is least used in the optimalsubset including the beam whose coverage includes the station S (stepE23). Remember that the optimal subsets are determined by the FIG. 3algorithm.

If the coverage including the station S has no free channel, then thesubsequent beam coverages from that nearest to that farthest from thecoverage C including the station S are examined to determine which ofthese coverages is produced by a beam having a free channel (steps E21and ET22). As soon as one of these coverages adjacent the coverage Cincluding the station S has been identified, a load transfer (step E22)is effected between the beam F associated with the coverage includingthe station S and the beam Fa including a free channel. As describedwith reference to FIG. 11 in the previously mentioned U.S. Pat. No.5,355,138, this transfer consists in reducing the size of the coverageof beam F including the station S and increasing commensurately the sizeof the coverage associated with the beam Fa including a free channel sothat the latter coverage can include the station S. As previously, ifonly one free channel is found in the adjacent coverage, then this freechannel is allocated to the station S. If plural channels are found inthe adjacent beam, then to the station S is allocated whichever of thesefree channels is least used in the optimal subset including the adjacentbeam (step E23).

By definition, the optimal subsets depend on the geometry of the beamcoverages and therefore on the configuration of the beams. The loadtransfer described with reference to step E22 modifies the coveragegeometry. Consequently, between the load transfer (step E22) andallocation to the station S of the free channel least used in theadjacent beam there is a step for determining optimal subsets ED usingthe algorithm shown in FIG. 3. The step ED redefines the optimal subsetsin dependance on modifications relating to the respective coveragesincluding the station S and associated with an adjacent beam having afree channel, following the load transfer (step E22).

Reverting to the other alternative at step ET20, if the station S is notincluded in any beam coverage, the invention offers a coveragereconfiguration step, as shown in step E24. As described with referenceto FIG. 5 of the previously mentioned U.S. Pat. No. 5,355,138, thisreconfiguration consists in selecting on the one hand the coveragenearest the station S and on the other hand the coverage with thelowermost surface on the ground. The control station SC runs asimulation for each of these two coverages to reduce the surface of thecoverages other than said each of these two coverages in order to freeup through simulation a fraction of the power of the satellite SA, andthereby to increase the size of said each of these two coverages inorder to include therein the station S. Whichever of these two coveragesthat can include the station S has the higher antenna gain is thenselected (step ET23). The steps ET21, E21, ET22, E22, ED and E23previously described are then carried out for the coverage now includingthe station S.

If neither the nearest coverage nor the lowermost surface coverage caninclude the station S, then the call setting-up request from the callingstation is queued (step EF).

As indicated in step E30, there is also provision for redistributingbusy channels in the beams so that each beam contains the same number ofbusy channels. This redistribution facilitates network management byreducing the number of beams in which all channels are busy. To thisend, if the number of busy channels in a given beam is greater than asubstantially mean number of busy channels per beam such that:

    E (CO/I),

where E denotes the integer part function, CO the number of busychannels in the network and I the number of radio beams in the network,then steps E21, ET22, E22, ED and E23 are applied to said given beam, sothat some stations in the given beam are assigned a channel of a beamadjacent said given beam in order to free up a channel in the givenbeam.

What we claim is:
 1. In a satellite telecommunication network comprisinga control station, a satellite for forming beams of radio channels underthe control of said control station, and plural earth stations fortransmitting beam channels to said satellite and receiving beam channelsfrom said satellite, same channels being re-used in different beams,themethod for allocating a free beam channel by said control station tocalling and called stations, in response to reception of a callsetting-up request message emitted by said calling station via asignalling channel of said satellite in said control station, saidmethod entailing simulation in said control station of one of first andsecond alternatives separately for each of the calling and calledstations, and then a final step,(a) said first alternative whereby atleast one free channel belongs to a beam forming a coverage includingeach of the calling and called stations, comprising the step ofallocating said at least one free channel to said each of the callingand called stations; (b) said second alternative whereby there is nofree channel on said beam forming said coverage including said each ofthe calling and called stations, comprising the following iterativesteps:(b1) searching for free channels in each of adjacent beams fromthat nearest to that farthest from said beam forming said coverageincluding said each of the calling and called stations therebyidentifying free channels, and (b2) in response to said free channelsidentified in each of said adjacent beams, allocating in a simulated wayone of said free channels identified in each of said adjacent beams tosaid each of the calling and called stations by transferring loadbetween said coverage including said each of the calling and calledstations and an adjacent coverage associated with each of said adjacentbeams by reducing in surface said coverage including said each of thecalling and called stations and increasing in surface said adjacentcoverage so that said each of the calling and called stations isincluded in said adjacent coverage, and determining optimal subsets ofbeams each comprising beams able to re-use the same channel, andselecting said one of said free channels identified in each of saidadjacent beams as that which is least used in one of said optimalsubsets of beams including each of said adjacent beams; and (c) saidfinal step comprising the step of emitting a call setting-upauthorization message from said control station to said calling stationas soon as said one of said free channels identified is allocated toeach of said calling and called stations.
 2. A method according to claim1 wherein said step of determining optimal subsets of beams includes thefollowing iterative steps of:constituting ##EQU4## separate subsets of Tbeams from I beams of said network, T being an integer initialized to 2and being incremented by 1 on each said iterative step up to at most I,and for each of said C_(I) ^(T) subsets of T beams calculating T levelsof isolation each equal to the ratio of an antenna gain of a main lobeof a respective one of said beams in each of said C_(I) ^(T) subsets ofT beams to the sum of the respective maximal antenna gains of secondarylobes of the other beams in each of said C_(I) ^(T) subsets of T beams,provided that a conditional relationship establishing that T levels ofisolation calculated for at least one of said C_(I) ^(T) subsets of Tbeams are more than a predetermined threshold is satisfied, said optimalsubsets of beams being made up of subsets of beams from C_(I)^(T).sbsp.L⁻¹ =(I!)/[(T_(L) -1)!(I-T_(L) +1)!] subsets of beams for eachof which [T_(L) -1] levels of isolation calculated are more than saidpredetermined threshold, T_(L) denoting an integer value assumed by Tfor which said conditional relationship is not satisfied.
 3. A methodaccording to claim 1 including in the simulation carried out in saidcontrol station, a third alternative whereby said each of the callingand called stations is not included in any beam coverage, comprising thesteps of:determining from beam coverages existing in said network acoverage nearest to said each of the calling and called stations and alowermost surface coverage; by iteration, for each of the nearest andlowermost surface coverages, reducing in size each of said coveragesother than said each of the nearest and lowermost surface coverageswhilst maintaining in said coverages stations which are active prior tosaid reception of the call setting-up request message, until said eachof the nearest and lowermost surface coverages can be increased toinclude said each of the calling and called stations with a respectiveantenna gain in said each of the nearest and lowermost surfacecoverages, and selecting one of said nearest and lowermost surfacecoverages into a selected coverage as a function of the higher of twoantenna gains respectively calculated for said nearest and lowermostsurface coverages to include said each of the calling and calledstations in said selected coverage, said step of selecting beingfollowed by one of said first and second alternatives and said finalstep.
 4. A method according to claim 1 comprising, in response to anumber of busy radio channels in one of the beams existing in thenetwork greater than the integer part of a ratio of a number of busyradio channels in said network to the number of beams existing in saidnetwork, executing said second alternative for each station covered bysaid one of the beams, when the number of said busy radio channels insaid one of the beams is greater than said integer part, so that to eachof said stations belonging to a coverage of said one of the beams isassigned a radio channel of a beam adjacent said one of the beams tofree up a radio channel in said one of the beams.
 5. In a satellitetelecommunication network comprising a control station, a satellite forforming beams of radio channels under the control of said controlstation, and plural earth stations for transmitting beam channels tosaid satellite and receiving beam channels from said satellite, samechannels being re-used in different beams,the method for allocating afree beam channel by said control station to calling and calledstations, in response to reception of a call setting-up request messageemitted by said calling station via a signalling channel of saidsatellite in said control station, said method entailing simulation insaid control station of one of first and second alternatives separatelyfor each of the calling and called stations, and then a final step,(a)said first alternative whereby at least one free channel belongs to abeam forming a coverage including each of the calling and calledstations, comprising the step of allocating said at least one freechannel to said each of the calling and called stations; (b) said secondalternative whereby there is no free channel on said beam forming saidcoverage including said each of the calling and called stations,comprising the following iterative steps:(b1) searching for freechannels in each of adjacent beams from that nearest to that farthestfrom said beam forming said coverage including said each of the callingand called stations thereby identifying free channels, and (b2)allocating to said each of the calling and called stations of one ofsaid free channels identified in each of said adjacent beams as soon asin an optimal subset of beams comprising all beams using the samechannel as said one of said free channels, each level of isolation equalto the ratio of an antenna gain of a main lobe of a respective beam fromsaid all beams in said optimal subset to the sum of maximal antennagains of secondary lobes of the other beams in said optimal subset isgreater than a given threshold; and (c) said final step comprising thestep of emitting a call setting-up authorization message from saidcontrol station to said calling station as soon as said one of said freechannels identified is allocated to each of said calling and calledstations.